The characteristics of a generally inaccessible body through which a fluid flows can in many cases be determined by adding to the fluid flow upstream of the body an injectant which modifies a property of the fluid and detecting, over time, the modified property downstream of the body. One particular example of this is in the field of medicine in which the onset of pulmonary edema or lung water is to be detected in vivo in advance of the lung water accumulation reaching the point where it severely affects the patient and is thus detectable through other less sensitive means.
Techniques of indicator dilution have been utilized for several decades in clinical research in an effort to detect the onset of pulmonary edema which may typically occur at times following serious surgery. Indicator dilution measurements have typically involved injecting into the blood a modifier of thermal, conductivity, radioactive or dye properties of the blood in the pulmonary artery with the indicator passing through the pulmonary vascular network where it may be detected downstream of the pulmonary vein, typically after passage through the left heart, where catheter mounted detection sensors may be readily placed or blood samples withdrawn for dye marker or radioisotope detection. This technique operates on the relative tendencies of the lung water mass to reversibly absorb or not absorb indicator as it flows through the pulmonary vascular network. In particular, the use of an indicator such as heat which is readily diffusable into the high specific mass of lung water is advantageously combined with a less diffusable indicator such as an appropriate conductivity modifier or dye marker.
The success of such techniques has been substantially limited by the complexity of the response of the pulmonary and cardiovascular system to different types of indicators as well as the difficulty of accessing living patient's organs in vivo for empirical analyses. Blood temperature, plasma solute ions and proteins, hematocrit and osmolality properties as well as heat diffusion paths other than through lung water of the full pulmonary circulatory system are felt to affect the sensed indicator magnitudes in ways which are not thoroughly understood or predictable. In addition, noise errors may appear in the detected signals such as from indicator recirculation. These tend to seriously affect the value of early lung water detection by indicator dilution and preclude the detection of lung water in absolute terms as opposed to relative values.